# 给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
#
# 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
#
#  示例 1：
# 输入：digits = "23"
# 输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
#
#  示例 2：
# 输入：digits = "2"
# 输出：["a","b","c"]
from typing import List


class Solution:
    def letterCombinations2(self, digits: str) -> List[str]:
        res = []

        return res

    def letterCombinations1(self, digits: str) -> List[str]:
        res = []
        n_chars_map = {
            '2': ['a', 'b', 'c'],
            '3': ['d', 'e', 'f'],
            '4': ['g', 'h', 'i'],
            '5': ['j', 'k', 'l'],
            '6': ['m', 'n', 'o'],
            '7': ['p', 'q', 'r', 's'],
            '8': ['t', 'u', 'v'],
            '9': ['w', 'x', 'y', 'z']
        }

        def dfs(index: int, single_res: str) -> None:
            if index == len(digits):
                res.append(single_res)
                return
            for ch in n_chars_map[digits[index]]:
                # single_res += ch
                dfs(index + 1, single_res + ch)
                # single_res = single_res[:-1]

        dfs(0, '')
        return res if len(digits) else []

    def letterCombinations(self, digits: str) -> List[str]:
        return self.letterCombinations1(digits)


if __name__ == "__main__":
    digits = '23'
    print(Solution().letterCombinations(digits))
